Correctly compute live variable information for physical registers

when an implicitely defined register is later used by an alias. For example:

         call foo
         %reg1024 = mov %AL

The call implicitely defines EAX but only AL is used. Before this fix
no information was available on AL. Now EAX and all its aliases except
AL get defined and die at the call instruction whereas AL lives to be
killed by the assignment.

llvm-svn: 10813